The invention relates to methods of and apparatus for detecting light. More particularly, the invention relates to light imaging using an array of photo detectors.
It is known to manufacture solid state light imagers or x-ray detectors using an array of rows and columns of photo detector elements. See for example U.S. Pat. No. 4,996,413, issued to McDaniel et al. on Feb. 26, 1991, which is incorporated herein by reference. Each element includes a photo diode, which converts photons to an electrical signal, and a transistor. Each photo diode has a capacitance associated therewith. The cathode of the photo diode in each element is connected to the source of the transistor in the element. The anodes of all the photo diodes are connected together and to a negative bias voltage -V.sub.b. The drains of transistors in respective columns are connected to respective column electrodes, and the gates of transistors in respective rows are connected to respective row electrodes.
To acquire an image from the array, the column electrodes are connected to a known, stable voltage V.sub.c. While the column electrodes are connected to the voltage V.sub.c, the row electrodes are connected to a voltage V.sub.on which is positive relative to V.sub.c. The photo diodes will be back biased. The transistors will conduct and a charge will be placed on each of the capacitances associated with the photo diodes. After the diodes have been charged, the row electrodes are connected to a voltage -V.sub.off which is negative relative to both V.sub.c and -V.sub.b to turn the transistors off and prevent them from conducting. The array is then exposed to light energy, such as produced by a scintillator which is exposed to x-rays. The photo diodes will conduct and the capacitance associated with each photo diode will be partially discharged. The amount of charge removed from the capacitance associated with each photo diode will depend on the intensity and duration of the light energy striking that particular photo diode. If the duration of light energy is the same for each of the photo diodes, as is the case when an x-ray exposure is made, then the amount of charge removed from the capacitance associated with the photo diodes will represent the intensity of the light energy striking the various photo diodes, and can be read by measuring the amount of charge required to restore each photo diode's capacitance. The charges measured for restoring the capacitances can therefore be used to create an image.